Conventionally, an image forming apparatus such as a copying machine and a facsimile has broadly adopted an electrophotographic method. The image forming apparatus adopting the electrophotographic method forms a toner image by developing a latent image that is formed on an image bearing member, and electrostatically transfers the toner image to a transfer material. The toner image thus transferred to the transfer material is heated and pressed in a fixing apparatus so that the toner image is fixed on the transfer material.
On the contrary, Patent Document 1 discloses that a resistive element having a positive temperature coefficient (PTC) characteristic is used as a heating element of a fixing apparatus, and a heating voltage is applied to the resistive element in a direction perpendicular to a carrying direction of a recording paper. The positive temperature coefficient is a characteristic in which a resistance value linearly increases as a temperature increases. In other words, as the temperature becomes high, the resistance value also becomes high.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2006-350241 (published on Dec. 28, 2006)
However, a temperature increases in an area where the recording paper does not pass, because heat does not transfer to the recording paper in the area. In the arrangement disclosed in Patent Document 1, such the state causes a resistance value to increase in the area where the recording paper does not pass, because the heating element has the PTC characteristic. In the state where the resistance value has increased in the area where the recording paper does not pass, when a current is applied perpendicularly to a carrying direction of the recording paper, a high resistance part where the recording paper does not pass is connected in series with a low resistance part where the recording paper passes. In such a series circuit, the same current flows in the high resistance part where the recording paper does not pass and in the low resistance part where the recording paper passes. Therefore, a heat value is higher in the high resistance part where the recording paper does not pass than in the low resistance part where the recording paper passes. This causes a problem in which a difference in temperature distribution becomes larger between the area where the recording paper does not pass and the area where the recording paper passes. This worsens endurances of members provided in the area where the recording paper does not pass. Further, it becomes difficult to obtain a uniform and high-quality fixed image on a wide-width paper that is processed immediately after processing of narrow-width papers at high throughput.